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It is not gloom and darkness but sunlight which drives people to kill themselves, according to new research. A study of suicides in Greenland has found that suicide rates increase sharply in summer, worsening with increased northerly latitude and round-the-clock daylight.

The counter-intuitive findings are reported in a paper by Karin Sparring Björkstén from the Karolinska Institutet in Sweden, co-authored with Danish researcher Peter Bjerregaard and Californian Daniel Kripke.

The three academics studied suicides in Greenland from 1968-2002, during which period there were apparently 1351 suicides and 308 homicides. The researchers compared suicide rates with homicides and also with local beer consumption, to see if there were any correlations. It could have been, for instance, that Greenlanders would drink more beer in the summertime and being boozed-up would lead them to top themselves - not the mere fact that it was summer.

But this wasn't the case. The study reports "peaks in June and troughs in the winter in all suicide cases... but not in homicides nor in consumption of beer".

Not only did more suicides happen in the summertime, there were more suicides the further north people lived - ie, the more sunlight they experienced. As one travels into the arctic in summertime, the sun stays in the sky for 24 hours a day. This seems to make people more likely to kill themselves more than they otherwise would.

"Suicides were more concentrated in the summer months north of the Arctic Circle than south of it, and most concentrated in North Greenland," write Björkstén and Co. North Greenland has constant daylight in summer.

Ruling out alcohol and possible associations with violence, the researchers were forced to speculate that in fact an excessive amount of sunlight makes people commit suicide. Citing other scientists' research, they say that this is actually well-established worldwide, which seems to rule out any connection between Greenlander suicide rates and the predicted appearance of Humvee-sized meat-eating armoured spiders in the region.

In spite of most studies showing spring and summer peaks, it is a wide-spread belief that the peak occurs in late autumn and early winter in relation to darkness. Spring and summer peaks have been demonstrated in the northern hemisphere in countries like West Greenland, Finland, Norway, Belgium, France, Italy, Japan, USA, Lithuania and Switzerland.

The southern hemisphere shows a mirror image: Spring and summer peaks in December for Chile, in September - October in South Africa, in November in Queensland and in October in Victoria. Studies from the equator are scarce, but an absence of seasonal variation has been reported from Singapore and also from places more distant from the equator like Los Angeles and Sacramento, California.

"Light is only one of many factors in the complex tragedy of suicide," say Björkstén and the others, "but this study shows that there is a possible relationship between light and suicide".

A preliminary draft of the paper, to be published in BMS Psychiatry, can be downloaded here. ®